In human, the eyes, which are almost perfect spheres, are contained by the bony orbits of the skull and project outwardly therefrom and are made rotatable therewithin through actuation of any one or more of the three pairs of agonist and antagonist oculomotor eye muscles: the four recti muscles and the two oblique muscles. Each eye has a pair of upper and lower lids to close it off from the environment. The process of closing the lids is mediated by a relaxation of the levator palpebrae muscle, coupled with a contraction of the obricularis oculi muscle, which occur during e.g. spontaneous blinking. A blink is bilateral, lasts about a quarter of a second and takes place throughout waking life once every few seconds. On the margins of the eyelid, there are eyelashes, or cilia. Totalling about two hundred in each eye, they have an average life of a few months.
The discoid glassy front of the eyeball, the cornea, is the principal image-forming surface thereof. In normal eyes, because of the large difference between the cornea refractive index and that of air, the cornea has an optical power of forty diopters (the number of diopters is given by 100/f, where f is the focal length in centimeters).
Sadly, defects of sight are common. The commonest problem is out-of-focusness or ametropia, which includes: hypermetropia, or long-sightedness; myopia, or short-sightedness; and presbyopia. The first defect can be corrected by increasing the dioptric power of the eye with convexo-convex lenses. The second defect, usually caused by an elongation of the eyeball, can be corrected by reduction of the dioptric power of the eye by means of a concavo-concave lens. Presbyopia affects virtually everyone starting from about forty years of age.
The prior art reveals that four methods have been developed to correct these conditions.
The first and most common sight correction method has been to provide spectacles consisting of a frame carrying a pair of optical lenses spacedly in front of the eyeballs. The spectacle frame rests on the wearer's nose and ears. Such spectacles are often the most comfortable and safest healthwise. However, because of the relatively large distance between each spectacle lens and the eyeball cornea, optical aberrations are introduced and there is a limitation on the overall visual field --the projection of the outside world onto the retina--. Also, because they are automatically noticed by nearby persons looking at the spectacled wearer, they tend to be a concern to aesthetically conscious spectacles wearing persons; moreover, athletes playing relatively rough sports such as soccer or basketball may find it difficult to keep their spectacles on their nose because of blows sustained from time to time during play.
An alternate embodiment of the first method of sight correction with spectacles, includes hydraulic lenses, wherein different amounts of fluid are forced between thin flexible glass plates. The fluid having a different refractive index than air, the dioptric power of the lens can be varied and adjusted by the wearer accordingly. Of course, such a spectacle assembly is quite complex and has not met wide approval among the general public.
The second method of sight correction consists in providing a smaller concave lens to fit flatly and directly against the (convex) cornea of each eyeball. These are called "contact lenses", because they engage physically with the eyeball flatly thereagainst. See for instance U.S. Pat. No. 4,850,689 issued 25 July 1989 to Martin. Optical Aberrations are therefore practically eliminated, and the lenses are concealed. Also, contact lenses can correct the irregular shape of the cornea in conditions of keratoconus, scarred cornea or other related conditions or irregular astigmatism. On the other hand, a myriad of complications are associated with contact lenses: periodic upkeeping, foreign body irritation, allergy reaction and particularly bacterial infections which could compromise the long term health condition of the eyeball.
A third method of sight correction, disclosed in U.S. Pat. No. 3,591,264 issued in 1971 to Forrest, tries to combine the advantages of the first and second above-noted methods while avoiding their drawbacks. In this patent, a small lens is edgewisely mounted to each lower eyelid, to project upwardly therefrom to come in register with the eyeball line of sight. This small lens is anchored to the lower eyelid by a pressure sensitive adhesive strip, so that it never engages the eyeball cornea although it is positioned very close thereto. The lens does not interfere with closing of the eyelids, and is concealed to all but those that take a close look at the eyes of the wearer. The field of view is very wide, almost as wide as for contact lenses.
The major weakness of the Forrest patent is its anchoring means to the eyelid, and more to the point, its outwardly upwardly inclined orientation. Indeed, use of such a lower eyelid dependent optical lens is quite limited in that it cannot be used for straight ahead vision of distant objects, since its general plane lies at an angle relative to the line of sight distant vision axis of the eye. Moreover, this lens could not be mounted to the upper eyelid, due to the particular type of anchoring means envisioned (the pressure sensitive adhesive strip), since it would eventually fall off from the upper eyelid by its own weight. Its general orientation--diverging upwardly and outwardly from the eyeball--further means that, due to its inherent weight, the lens will tend with time to pivot away from the eyeball. This downward pivot motion of the lens will obviously decrease in a progressive fashion the relative acute angle made between the plane of the lens and the near-sight vision line of sight of the eye, thus in effect changing the dioptric power of the lens and consequently, blurring the vision of the wearer with time. Moreover, this progressive downward motion of the lens will bias the latter to eventually fall off from the lower eyelid, or at least will induce load-bearing forces about the lower eyelid free edge, which will shear the lower eyelid anchoring tissues and eventually damage same.
The last method of sight correction consists in invasive surgery of the eyeball itself, either by incision of the cornea with knives or by photo-refractive keratectomy with the lasers (Excimer Laser, Picosecond Nd: YAG-Laser, etc.), to permanently correct the visual impairment. The incisional method has been pioneered by a Soviet physician, Konstantin Feodorov, in the case of myopia, through a technique known as radial keratotomy. Radial keratotomy requires deep incisions of the cornea. However, some opthalmologists have expressed concerns about such an operation, because of the non-negligeable risks of harming the eyeball, for example inducing perforation and infection thereof. There is a potential risk of inducing total blindness. As to the photo-refractive method with the lasers is still under investigation: It entails also some risks in addition to the fact that it too much expensive.
Materials that may be used for some of these correcting lens include: polymethyl-methacrylate, polypropylene, or polyethylene-terephthalate (known under the registered trademark DACRON).